Dry Fasting: What the Evidence Shows

What "Dry Fasting" Means (and How It Differs From Water-Only Fasting)

Dry fasting means abstaining from both food and all fluids, including water. That is the single difference from water fasting, which permits water throughout the fast, and from the broader landscape of fasting types described in what is fasting. A 2024 international consensus paper on fasting terminology defines dry fasting as "a fasting regimen during which a voluntary abstinence from all foods and beverages, including water, is practiced for a certain period of time," and separately terms fasting of 2 to 3 days "short-term fasting" and fasting of 4 or more consecutive days "prolonged fasting" (Koppold et al., 2024) [1]. This page uses that same short-term and prolonged split to separate the two kinds of dry fasting discussed below.

The Human Evidence Is Almost Entirely About Ramadan, Not Multi-Day Dry Fasting

Ramadan observance is a daily dry fast: no food or water from dawn to sunset, which can reach up to 20 hours a day in summer at higher latitudes (Malik et al., 2021) [3], with normal eating and drinking overnight, repeated for a month. That pattern, "intermittent dry fasting," is behind nearly all of the human research on this page. "Prolonged dry fasting," no food or water at all, around the clock, for multiple consecutive days, is a different and far less studied practice. Online content that promotes multi-day dry fasting often implies it draws on the Ramadan research. It does not. The two practices differ enough in duration and continuity that a finding from one should not be read onto the other.

What the Daily (Intermittent) Dry Fasting Research Shows

Kidney Function, Especially in Chronic Kidney Disease

Trials of fasting as a treatment for chronic kidney disease are a separate area and are not summarized here; the citations below describe fasting considerations and guidance for people who already have the diagnosis, not evidence that fasting treats, manages, or improves kidney disease. A review of Ramadan fasting in people with chronic kidney disease noted a paucity of evidence and suggested that everyone with chronic kidney disease be considered high or very high risk for fasting, that people with stable stage 1 to 3 disease may be able to fast with careful monitoring, and that people on dialysis should not be encouraged to fast (Ahmad and Chowdhury, 2019) [2]. A 2021 paper written for healthcare professionals gives practical guidance along the same lines: people with chronic kidney disease who choose to fast during Ramadan need individualized assessment, risk is not uniform across the condition's stages, and people in the high and very high risk tiers should be encouraged to explore alternatives to fasting (Malik et al., 2021) [3]. Neither paper reports a general-population kidney-function risk from daily dry fasting; both are specifically about an already-diagnosed patient population.

Kidney Stones: Most Studies Found No Change, Two Found an Increase

A systematic review including 10 observational studies and 9,906 participants looked at whether fasting is linked to kidney stones, specifically renal colic. Among the studies that looked at Islamic Ramadan fasting specifically, 7 of 9 found no change in renal-colic incidence tied to the fasting month, while 2 found an increase (Kirubarajan et al., 2021) [4]. The review's authors concluded that, on the available evidence, it is unlikely that fasting significantly increases the risk of kidney stones, and that physicians should counsel higher-risk patients on safe fasting practices; all 10 included studies were observational. Both results are stated here alongside the authors' overall conclusion.

Hydration, Metabolism, and Circadian Timing During a Different Daily Dry Fast (Bahá'í Fasting)

A prospective, exploratory cohort study followed 34 healthy Bahá'í volunteers (15 women) through the Bahá'í fasting month: 19 consecutive days of no food or water from sunrise to sunset, with daily fasting windows of about 10.9 to 12.1 hours at the Berlin study site. Researchers measured hydration and metabolic markers before fasting, in the third week of fasting, and 3 weeks and 3 months afterward. Serum osmolality and 24-hour urine osmolality (n=34) decreased during fasting but remained largely within the physiological range and returned to pre-fasting levels during night hours. Body mass index, total body fat mass, and resting metabolic rate decreased during fasting, while body cell mass and body water appeared unchanged. In a subgroup of 16 participants, the circadian phase, estimated from blood-cell gene activity, shifted 1.1 hours earlier during fasting and returned to pre-fasting values 3 weeks after fasting; most changes were no longer detectable 3 months later. The study's authors read these results favourably, as indicating that this fasting pattern was safe for their healthy volunteers and had no negative effects on hydration (Koppold-Liebscher et al., 2021) [7]. This is one uncontrolled cohort study of 34 healthy adults, so it cannot show that dry fasting is safe for anyone else, and its daily fasting window of about 11 to 12 hours is shorter than Ramadan fasting can reach and far shorter than the 5-day dry fast discussed below.

Prolonged (Multi-Day) Dry Fasting Has Almost No Evidence Beyond One Research Group

Beyond Ramadan's daily pattern, controlled human data on dry fasting lasting multiple consecutive days is essentially absent. The only published physiological studies found covering this are two papers from a single research group, both reporting on the same ten participants during one supervised 5-day dry fast: the first paper tracked changes in hormones, blood markers, and body-weight components during the fast (Papagiannopoulos-Vatopaidinos et al., 2020) [5], and a second, later paper from the same group examined hypoglycemia-compensation mechanisms during dry fasting (Papagiannopoulos-Vatopaidinos et al., 2025) [6]. This is one small study, n=10, reported twice, 5 days, medically supervised, from one research group, and it has not been replicated by other researchers. Ten people studied once is not a basis for a general safety or effectiveness claim. The first study reported an average body-weight loss of 7.0 kg over the 5 days and broke it down by route of loss: urine 52%, insensible water loss 32%, respiratory gases 11%, stool 5% (Papagiannopoulos-Vatopaidinos et al., 2020) [5]. It did not measure body fat, so it does not show how much of the loss was fat. By day 5 the study reported rises in cortisol (495%), C-reactive protein (167%) and uric acid (103%); the second paper reported falls in insulin and insulin resistance and a rise in LDL cholesterol. The authors interpreted their findings favourably and proposed therapeutic uses that the studies did not test.

Why Dry Fasting Raises Dehydration and Electrolyte Risk Faster Than Water-Only Fasting

Water-only fasting allows the body to keep drinking even while it is not eating; dry fasting removes that option entirely. With no fluid intake at all, the body has nothing to draw on once its own water reserves are used, so warning signs of dehydration and electrolyte imbalance can develop faster, and with less margin for error, than during a water-only fast. This page does not give guidance on what to do about that risk at home. See electrolytes during a fast for the signs of a developing problem, and how to break a fast safely for how to end a fast carefully. Neither page's guidance is restated here, and nothing on this page substitutes for a clinician's advice on fluid or electrolyte management during any fast, let alone a dry one.

Who Should Not Attempt Dry Fasting Without Talking to a Clinician First

Talk to a clinician before any fast, of any length, and especially before a dry fast, if you have chronic kidney disease, manage diabetes or blood pressure with medication, are pregnant, are under 18, or have a history of an eating disorder. Dry fasting removes the option of drinking water if something starts to feel wrong, which is why these general fasting cautions carry more weight here than on a water-only fast, and why multi-day dry fasting specifically, given how little it has been studied, is not something to try without medical guidance regardless of which of these groups applies to you.

How Dry Fasting Compares to Water-Only Fasting

Water fasting and prolonged fasting cover the fuller duration-staging and research discussion for water-only fasts, where water is permitted throughout; that staging and research is not repeated here. The one difference that matters for this page is fluid intake: allowed throughout a water-only fast, not allowed at all during a dry fast, which is why the dehydration and electrolyte risks above are discussed separately, and more cautiously, here than on those pages.

Authors and Funding of the Cited Studies

• Koppold et al., 2024 [1]: funded by Immanuel Hospital Berlin, with publication costs covered by the Familie Deutsch Stiftung Naturheilkunde. The declaration of interests lists consultancies, company roles, board memberships, or equity for many of the paper's 44 authors, including Valter Longo, who holds an equity interest in L-Nutra Inc., a company that markets and sells the fasting-mimicking diet (100% of that equity is assigned to the non-profit foundation Create Cures). The citation's "et al." includes two co-authors relevant to this page, Valter Longo and Ioannis-Eleemon Papagiannopoulos-Vatopaidinos, an author of the two dry-fasting papers cited above [5][6].
• Ahmad and Chowdhury, 2019 [2]: the authors received no financial support for the research, authorship, or publication of this article, and declared no conflict of interest.
• Malik et al., 2021 [3]: the authors declared no conflicts of interest.
• Kirubarajan et al., 2021 [4]: the authors reported no competing personal or financial interests related to this work.
• Papagiannopoulos-Vatopaidinos et al., 2025 [6]: the authors received no funding from any public, commercial, or not-for-profit funding agency, and declared no conflicts of interest.
• Koppold-Liebscher et al., 2021 [7]: funded by the German Bahá'í Research Fund (Stiftung für Bahá'í Studien), with the funders not involved in designing or conducting the study. The authors declared the research was conducted without any commercial or financial relationships that could present a conflict of interest.
• Disclosures are listed where the full text of the paper was available to us; a study not listed here is not a statement that its authors have no ties.

Safety and Cautions

This is general information, not medical advice. The human evidence on dry fasting is lopsided: the daily Ramadan pattern has a real research base, with real disagreement inside it on kidney-related risk, while prolonged, multi-day dry fasting rests on one small study, n=10, 5 days, medically supervised, reported in two papers from one research group, not replicated. Talk to a clinician before fasting of any length if you are pregnant, under 18, manage diabetes or blood pressure with medication, have kidney disease, or have a history of an eating disorder. Stop and seek care for any severe or concerning symptom. For the broader pooled evidence on fasting generally, see what the research shows about fasting.